As an intensive property, a force cannot be measured directly. Force sensors make it possible to estimate the force applied via the deformation or the displacement of a proof body.
A force sensor is a device which brings together both electronics and mechanics. It makes it possible to convert a force, that is to say a physical vector quantity into an electrical quantity. To do this, there are various technologies and physical principles of sensors for measuring force, whether in the form of forces or moments. In particular among the sensors that can be cited there are those constructed by using mechanical deformation gauges connected to an appropriate electronic bridge, generally a Wheatstone bridge. This type of sensor is the one most commonly encountered, both in the industrial environment and in the scientific literature. For all that, this principle does present a certain number of drawbacks, among which the measurement noise and the phenomena of lifting of the gauges on the proof body of the sensor can be cited.
Another physical principle that makes use of resonant mechanisms is also the basis for a few force sensors. The resonant sensors use the measurement of the frequency variation of mechanical resonances of a structure by means of transducers to estimate a force.
Currently, the force sensors using the resonant structure principle do not make it possible to directly and simultaneously measure all the components of the force torque applied to the proof body of the sensor. For the moment, there are unitary resonant sensors which remain limited to measuring a single force component (longitudinal or transverse force). The measurement of some other components of the force torque can be done only by adding to the host structure other unitary sensors whose spatial configuration makes it possible to measure force components in other directions. The dynamic and simultaneous reconstruction of all the components of the force torque is not therefore direct.
In particular, resonant force sensors with geometries of “beam” type are known, produced with configurations of one, two or even three parallel beam type, as described in particular in the documents by A. Cheshmehdoost and B. E. Jones, Design and performance characteristics of an integrated high capacity DETF-based force sensor, Sensors and Actuators A: Physical, 52(13): 99-102, March 1996 and by T. Fabula, H. J. Wagner, B. Schmidt, and S. Buttgenbach, Triple-beam resonant silicon force sensor based on piezoelectric thin_lms, Sensors and Actuators A: Physical, 42(13):375-380, April 1994. These offer advantages over the so-called “single-beam” configuration cases, for example, better quality factor, and greater sensitivity. This type of structure has been used to measure a single component of the force, generally that whose direction is aligned along the axis of the beam. A document by C. Barthod, Y. Teisseyre, C. Ghin, and G. Gautier, Resonant force sensor using a PLL electronic, Sensors and Actuators A: Physical, 104(2):143-150, April 2003, describes non-axial force measurements in which the structures use transformation mechanisms. However, these mechanisms are expensive and complex to implement.
One technical problem to be solved is therefore how to produce a resonant force sensor that can measure the force components in all six dimensions, that is to say the three dimensions of force and the three dimensions of torque, that is also reliable and economical.